Operational amplifiers are widely used in electronic applications. A typical operational amplifier includes an output terminal, for producing an output signal which is proportional to the difference between two input signals applied to first and second input terminals. Mathematically, operational amplifiers may be characterized by the relationship V.sub.out =A(V.sub.in.sup.+ -V.sub.in.sup.-) where V.sub.out is the output voltage of the operational amplifier, V.sub.in.sup.+ and V.sub.in.sup.- are the two differential input voltages of the operational amplifier and A is the gain of the operational amplifier.
U.S. Pat. No. 4,963,834 to Yukawa illustrates an operational amplifier as described above, including an input differential stage to which the differential input terminals are connected, two folded cascode stages connected to the differential input stage, an inverting amplifier including two transistors for receiving outputs of the first and second cascode stages, a current mirror inserted between the folded cascode stage and one of the two transistors, and an output terminal connected to a connecting point of the two transistors. One of the two transistors is driven by the output of the second folded cascode stage, and the other of the two transistors is driven through the current mirror by the output of the first folded cascode stage, so that an amplified output is supplied from the output terminal. Other operational amplifiers are described in U.S. Pat. No. 4,797,631 to Hsu et al. and U.S. Pat. No. 4,958,133 to Bazes.
A special type of operational amplifier is the fully differential operational amplifier. A fully differential operational amplifier includes a pair of differential output terminals in addition to a pair of differential input terminals. Accordingly, the mathematical relationship for a fully differential amplifier may be expressed as V.sub.out.sup.+ -V.sub.out.sup.- =A(V.sub.in.sup.+ -V.sub.in.sup.-), where V.sub.out.sup.+ and V.sub.out.sup.- are the output voltages of the differential output terminals. Fully differential amplifiers are widely used in many modern analog sampled-data applications. These applications include oversampled analog-to-digital converters, multistep analog-to-digital converters and switched capacitor filters. Also, many of these high speed applications require a high DC (open loop) gain, a high slew rate limit and high common mode rejection.
The need for faster fully differential operational amplifiers has resulted in the use of a current steering technique, also referred to as a folded cascode, for high performance applications. Fully differential operational amplifiers typically include a differential input circuit and a pair of folded cascode amplifiers. Unfortunately, it has been found that this type of fully differential amplifier is slew rate limited, does not have a symmetric slew rate and may require auxiliary circuits to reject common mode variations and provide common mode biasing.
A fully differential FET operational amplifier of the folded cascode type is described in U.S. Pat. No. 4,658,219 to Saari. Saari includes a second cascode transistor having its source connected to the source of a first cascode transistor, and its drain connected to the source of a pull down transistor, so that the pull down transistor feeds part of the signal to the output node as a cascode device. With this arrangement, the width-to-length ratios of the transistors connected to the output node can be substantially reduced for an increase in the output impedance, while the output current drive capability of the amplifier is maintained. This results in an amplifier with increased open loop gain and reduced distortion. Other fully differential operational amplifiers are described in U.S. Pat. Nos. 4,656,437 to Saari; 4,667,165 to De Weck; 4,668,919 to De Weck; 4,749,956 to Torelli et al.; 4,933,644 to Fattaruso et al.; and 4,965,468 to Nicollini et al.
Notwithstanding the extent of developmental activity noted above, modern applications of fully differential amplifiers require ever increasing DC gain, improved common mode rejection and higher symmetrical slew rate.